This invention relates to improvements in optical fiber manufacture. More specifically, the invention relates to a method and apparatus for providing improved fiber drawing from an optical preform.
In the drawing of optical fibers from an optical preform, it is necessary to heat a portion of the preform to its "drawing temperature." A filament of glass is pulled from the heated portion of the preform to form an optical fiber. It is possible to give the fiber the desired light transmitting properties by carefully regulating the diameter of the fiber drawn from the preform. In order to provide an optical fiber having uniform light transmission characteristics along its length, it is imperative that the diameter of the fiber be maintained constant along the length of the fiber. Additional factors such as the fiber drawing temperature and tension, the rate of drawing and the protection of the drawn fiber, all affect the optical characteristics of the drawn fiber.
The fiber drawing tension significantly affects the optical transmission properties of the final glass fiber. Since drawing temperature affects both optical and mechanical properties of the drawn fiber, optimum drawing temperature must be employed to obtain a fiber having the desired properties. The drawing temperature is associated with other fiber drawing parameters, such as drawing speed, preform and fiber diameter, and preform feed speed. Therefore, fiber drawing tension, which is mainly dependent on the drawing temperature, should be carefully controlled.
Very low loss optical fibers can be drawn at a drawing tension approaching 50 grams. On the other hand, low drawing tension, for instance of less than 5 grams, may be maintained during drawing of long lengths of high strength fiber. The low drawing tension is achieved by utilizing a high drawing temperature. However, high drawing temperature causes a high degree of silica vaporization. The upper limit of the drawing temperature with regard to optimum drawing tension depends on drawing conditions. The quality of the drawn fiber, that is, its mechanical and optical properties, will also be dependent on the degree of contamination of the material of the fiber. Therefore, it is imperative that the fiber be manufactured under conditions which avoid, to the largest possible or feasible extent, introduction of impurities into the material or the fiber. Moreover, the surface of the drawn fiber should be as perfectly smooth as possible since any irregularities or defects in such surface adversefly affect the optomechanical properties of such fiber and especially degrade the strength of the fiber.
Therefore, it is customary to use preforms which contain a minimum amount of impurities, if any, and to conduct the fiber drawing operation in an environment which is free of dust and other contaminants. Yet, such contaminants may be introduced into the material of the preform or of the fiber by the heat source which heats the portion of the preform from which the fiber is being drawn. Therefore, it is desirable to use a relatively "clean" heat source, that is, a heat source which will not introduce any significant amount of contaminants into the material of the preform of fiber.
A relatively clean economical heat source is an oxyhydrogen torch which produces a flame as a result of combustion of purified hydrogen with purified oxygen. However, experience with conventional heat sources of this type has shown that the oxyhydrogen flame produced by the same is subject to flucuations and that, consequently, the heat flux into the preform is irregular. This, in turn, means that the visosity of material of the portion of the preform from which the fiber is being drawn varies from point to point with attendant variations in the flow rates of the material at such points and, consequently, in relatively poor control of fiber dimensions.
This problem is ameliorated when, in accordance with conventional techniques, at least two oxyhydrogen torches are so arranged around the axis of the preform and of the fiber that their flames are aimed substantially radially against the portion of the preform from which the fiber is being drawn, and when relative angular displacement about the aforementioned axis is effected between the preform/fiber combination and the oxyhydrogen torches, especially when the combination is rotated about its longitudinal axis. However, even in such a construction, the above-discussed problem of the flame fluctuations and their effect on the quality of the drawn fiber is not completely eliminated.